The field of the invention is hemoglobinopathies.
A number of hemoglobinopathies are associated with a decreased affinity of oxygen for hemoglobin (Hb). Examples of such hemoglobinopathies include sickle cell disease (SCD) and sickle cell trait; the thalassemias; Hb-C, Hb-D, Hb-E, Hb-H, Hb-I, Hb-O-Arab, and Hb-Kansas disorders; and mixed heterozygous combinations of any of these. Perhaps the most extensively studied hemoglobinopathy is sickle cell disease (SCD), which results from a mutation at the sixth amino acid from the N-terminus of the .beta.-polypeptide chain of hemoglobin (Hb-S). This mutation replaces the negatively charged amino acid glutamate (encoded by GAG) with a neutral, hydrophobic residue, valine (encoded by GUG). At physiologically relevant concentrations (e.g., about 0.2 g/cm.sup.3), and under certain conditions such as hypoxia, Hb-S molecules aggregate into multi-stranded polymers, forming a viscous or solid-like gel. This polymerization of Hb-S within red blood cells (erythrocytes) distorts the erythrocytes into a variety of abnormal shapes, some of which resemble sickles. Polymerization of Hb-S also causes erythrocytes to become more rigid, decreasing the cells ability to traverse the narrow vessels of the microvasculature.
Individuals who carry one gene encoding normal adult hemoglobin (Hb-A) and one encoding Hb-S are said to have sickle cell trait. Such heterozygotes rarely suffer clinical symptoms, but can of course pass the mutant gene on to their offspring. Individuals who are homozygous for Hb-S or a combination of Hb-S and Hb-C are said to have sickle cell disease (SCD); these individuals are referred to herein as "SS." Adults whose RBCs contain normal hemoglobin are referred to herein as "AA."
Clinically, SCD is associated with chronic hemolysis, which can lead to anemia and jaundice. SCD is also associated with acute episodic vaso-occlusive crises, known as sickle crises. During such a crisis, the microvasculature can become transiently or permanently occluded, and nearly every organ of the body can be damaged as a result of the decreased oxygen supply. Such organ damage is the major cause of the mortality and morbidity associated with SCD. Infarction (i.e., necrosis of tissue due to insufficient blood supply) of bone, spleen, kidney, and lungs is particularly common, and results in severe pain that can last for several days.
Conventional methods for treating hemoglobinopathies such as SCD include performing blood transfusions on anemic patients to produce a hemoglobin level of 10 g/dl and/or achieve a total hematocrit concentration of 30%. Other methods involve using hydroxyurea to reduce erythrocyte sickling by increasing the level of fetal hemoglobin (Hb-F) in erythrocytes (Charache et al., 1995, N. Eng. J. Med. 332:1317-1322; Goldberg et al., 1990, N. Eng. J. Med. 323:366-372). Using this method, several weeks of treatment are needed to increase the level of Hb-F; this method, therefore, is not particularly useful for responding to a sickle crisis. Additional treatment methods include providing supportive therapies, such as oxygen therapy, analgesics, and hydration. Such supportive therapy is provided to the patient until the crisis is resolved, which usually takes several days.
For any given hemoglobinopathy, the affinity of the patient's hemoglobin for oxygen can be measured by generating an oxyhemoglobin dissociation curve (ODC) characteristic of the patient's total hemoglobin. This mathematical curve can be generated by plotting the percent oxygen saturation (S.sub.a O.sub.2) of the total hemoglobin on the y axis versus the partial pressure of oxygen (P.sub.a O.sub.2) in mm Hg over a wide range of oxygen pressures (e.g., 0 to 100 mm Hg) on the x axis (see, e.g., Bunn and Forget, Hemoglobin: Molecular Genetics and Clinical Aspects, 1986, W. B. Saunder, Publisher). The P.sub.a O.sub.2 at which half-maximal oxygen saturation of total hemoglobin occurs is termed the P.sub.50 value. Hemoglobin that has a decreased ability to bind oxygen is characterized by a rightward shift in the ODC, relative to the ODC obtained with normal adult hemoglobin (Hb-A); this can alternatively be expressed as an increase in P.sub.50, compared to Hb-A. Normal hemoglobin at 37.degree., PaCO.sub.2 40 mm Hg, pH 7.40, and isotonic conditions has a P.sub.50 of approximately 26 mm Hg, while Hb-S has a P.sub.50 of approximately 37 mm Hg. The P.sub.50 value of a given patient's hemoglobin can be measured readily by commercially available equipment, such as the HEMOX-ANALYZER.TM. automatic blood oxygen dissociation analyzer (TCS Medical Products Company, Huntingdon Valley, Pa.).